Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case hardened surface. Nitriding is most commonly used on low-carbon, low-alloy steels, however, during recent years, also higher alloyed steels have been nitrided with advantageous results.
The main nitriding methods used today are: gas nitriding, salt bath nitriding, and plasma nitriding, which are named after the medium used to donate nitrogen.
Nitriding typically imparts a high surface hardness which promotes high resistance to wear, scuffing, galling and seizure. Fatigue strength is increased mainly by the development of surface compressive stresses.
Nitriding is often performed at elevated temperature and is therefore typically ended by a cooling or quenching step in which the steel product is cooled down. Fast quenching after nitriding will increase the solution hardening effect from the entrapped nitrogen but this effect is proportionally small compared to the precipitation hardening effect derived from the formation of hard nitrides between alloying elements and nitrogen in the steel surface. Alloying elements such as Cr, Al, V, Ti and Mo forms hard nitrides in steel during nitriding and the level of such alloying elements in the steel has a huge impact on the nitriding result in terms of hardness, wear resistance and fatigue strength. Quenching oils and heat treatment fluids are designed for rapid or at least controlled cooling of steel or other metals as part of a hardening, tempering or other heat-treating process, such as nitriding.
Typical applications include gears, crankshafts, camshafts, racks, pinions, axles, races, drive shafts, center pins, cylinder blocks for hydraulic motors, vanes for pumps, piston skirts, chain components, slideways, cam followers, valve parts, extruder screws, die-casting tools, forging dies, extrusion dies, firearm components, injectors, plastic-mold tools, conveyor guides, etc.
Due to the typical beneficial properties of nitrided materials, they are often used in applications where the surfaces are exposed to mechanical contact with other solid or liquid objects, in particular, in moving contacts. In such applications, low friction and wear resistance are of interest. Lubrication is the standard way to address friction and wear problems. Depending on application, liquid and/or solid lubricants can be used. Liquid lubricants are the preferred choice when long service life, serviceability, corrosion protection, cleaning and cooling are all important. Solid lubricants are used in special cases where the use of liquid lubricants is not an option, due, for instance, to thermal conditions or surrounding environment. Solid lubricants are especially effective in controlling wear in highly loaded sliding contacts and hence are often used in applications being exposed to wear. There are several methods of applying such solid lubricants. Many such methods are based on the application of a paste or liquid containing dispersed solid lubricants onto the surface to be covered, followed by a heat treatment and/or mechanical treatment to remove the binding materials in the paste or liquid, causing the solid lubricant to bind to the surface of the article to be lubricated. However, without having been chemically bonded to the surface, solid lubricants are poorly retained and readily detaches from the surface. As a result, polymer-bonded solid lubricant coatings are most common in practice, including well-known commercial products from Dow Corning, Klueber, Henkel and many others. In these products, a thermoset, UV-set or oxidation-drying polymer binder is used to retain the solid lubricant on the surface. To apply the coating after the nitriding, the surface has first to be cleaned, then coated in a separate step, and then finally cured.
In the case of nitrided objects, such heating and/or mechanical treatment and/or cleaning may influence the composition and properties of the surface of the nitrided object itself. Heating at a low nitrogen potential may e.g. cause de-nitriding of the objects surface and heat treatment and mechanical interaction may alter texture, hardness, etc. of the nitrided object.
Another common way of manufacturing solid lubricant coatings is by means of physical vapor deposition (PVD), plasma-assisted chemical vapor deposition (PA-CVD) and similar vacuum processes, whereby solid lubricants are embedded into a hard coating—such as diamond-like carbon—matrix. This technology is used, in particular, to manufacture products such as Balinit C (Oerlikon), MoST (Teer Coatings) and others. Prior to the PVD (or PA-CVD) coating, too, the surface has to be thoroughly cleaned, and then coated in a separate step.
Nitrided steel articles can also be CVD coated by certain solid lubricants in a separate processing step. This might produce a tribological effect. For instance, one might produce MoS2 and WS2 coating by a CVD process reacting volatile metal carbonyl complexes, Mo(CO)6 and W(CO)6, with mercaptanes or organic sulfides, such as dimethylsulfides. Unfortunately, coatings so produced often tend to be fluffy and exhibit poor adhesion to the substrate. Possible reasons may be found in contamination or gas adsorption on the nitrided surface before coating or in surface modifications during cleaning procedures.
In all of the abovementioned cases, increased process complexity adds to logistic and manufacturing costs.